1. Field of the Invention
The present invention relates to estimating the call capacity in wireless communication systems.
2. Description of the Related Art
Recent developments in wireless communication systems have focused on increasing the capacity of wireless radio links. While these advances allow more calls to be processed at each base station, they may produce a situation where the processing capacity of a base station, rather than the radio link capacity, limits overall system performance.
In wireless communication systems, call control functions are typically performed by a single processor located in a base station. These call control functions tend to be transient events involved with call setup and handoff. For example, these call control events include processes executed by the call control processor to enable setting up of a call, handing off a call from one base station to another, and releasing a call. The steady-state traffic signals involved with a call-in-progress are not considered call control functions.
It is common to define xe2x80x9cprocessor occupancyxe2x80x9d as the fraction of time a processor is busy. For example, if the processor is busy performing operations for N seconds out of a T second period, the processor occupancy (expressed in percent) is (N/T)* 100.
The processor occupancy can be thought of as consisting of three components: no-load occupancy; call control processor occupancy and the occupancy due to operations; administration and maintenance (O, A and M). Of course, all processors have overhead associated with running the operating system, transferring data, etc. Thus, it is useful to define xe2x80x9cno-load processor occupancyxe2x80x9d as the processor occupancy that is not directly a result of call processing tasks. In other words, the no-load processor occupancy is the processor occupancy when no calls are being processed (i.e., under a so-called no-load condition). Similarly, xe2x80x9ccall control processor occupancyxe2x80x9d is defined as the processor occupancy due to the above-described call control functions.
xe2x80x9cO, A and M processor occupancyxe2x80x9d is defined as the processor occupancy due to the O, A and M functions. Because of the real time needs of call control functions, the operating system in the processor is typically preemptive with static or dynamic priorities, meaning that certain events must wait until other, higher priority events execute. Processes that execute O, A and M functions are assigned priorities lower than those assigned to call control processes, because O, A and M tasks typically have loose delay requirements, meaning that these tasks may be delayed a relatively long time. Processes that are responsible for operating system specific functions and which contribute to no-load occupancy have extremely tight delay requirements and are assigned priorities higher than those assigned to call control processes. As a consequence of the preemptive nature of the operating system, O, A and M processes cannot execute whenever there is any operating system (i.e., no-load) or call control processes that are runnable.
Therefore, from the point of view of call control functions, the relevant occupancy components that contribute to call processing delays are those due to no-load and call control. Thus, in subsequent discussions, xe2x80x9ctotal processor occupancyxe2x80x9d will be understood to be the sum of no-load processor occupancy and call control processor occupancy. The call control processor occupancy may be calculated if the set of all call control events, their rates, and their respective processing times is determined.
Typical call control events which contribute to the processor occupancy of a base station will now be discussed. For the purposes of discussion and for ease of description, advanced mobile phone service (AMPS) and time division multiplex access (TDMA) call control events will be primarily described. However, the following call control events are generic and intended to be representative of like events in other systems, and of like events in different AMPS and TDMA implementations from different manufacturers. All of the following call control events contribute to the processor occupancy of a base station, each call control event having an associated processing time
Call events may be grouped into the following eight categories, for the purposes of description: mobile originations, mobile terminations, call release, handoffs out of a base station, handoffs into a base station, pages, registrations, and locates. These eight general categories will now be more specifically described.
The phrase mobile originations refers to calls originated by a mobile unit. This call event includes all of the messages involved in setting up a call. For example, this event typically includes messaging to and from the base station which detects a call attempt, messaging to and from the mobile switching center (MSC) which may determine the channel to be assigned and messaging to and from the mobile which results in the mobile tuning to the assigned channel.
The phrase mobile terminations refers to calls originated in the PSTN and terminating at a mobile unit. This call event includes all of the messages involved in establishing such a call.
Call release, the ending of a call, may be either mobile-initiated or forced by the MSC.
Handoffs out of a base station occur when a mobile served by a base station moves into an area better served by another base station. Handoffs into a base station occur when a mobile moves into the area better served by the base station. Both of these handoff events are included when calculating the processor occupancy.
Pages are messages transmitted from a base station to all mobile stations within its coverage area (i.e., cell).
Autonomous registrations for AMPS and TDMA systems occur when a mobile unit xe2x80x9cannouncesxe2x80x9d its presence to the nearest base station (and thus to the MSC). Once this presence has been registered with the base station""s MSC (and with the mobile unit""s home area MSC, if applicable), mobile terminated calls can find the particular mobile unit for which they are intended.
The final class of call control events are those used in the coordination of measurement activity prior to handoffs, generically referred to as locate events. Such locates are typically used to find the best cell, and/or antenna face within a cell, to serve an existing call. Locate events may be generated within the cell (Self Locate) or received from neighbor cells (Neighbor Locate Requests Received). Locate events occur in both AMPS and TDMA systems, though the manner in which they occur is different between these systems Having thus defined several classes of call control events, the processing time for each event may be measured for a particular processor, and for both AMPS and TDMA systems. The measurement times include only the actual processing time spent, and do not include elapsed time spent waiting for responses (either from the mobile unit or the MSC). These measurements may be single thread (single call) measurements made in an unloaded system using a measurement tool suited for such a purpose.
Call processing times may change with the addition of features in a particular system, and with each new software release for the system. Hence, measuring these processing times for each new release/feature may be made part of the standard laboratory testing program
In both AMPS and TDMA systems, when a specified call control event occurs a counter may be incremented. The number thus accumulated may be used to calculate the rates of the above-described call control events. Such counts are known in the wireless art, and detailed descriptions of these counts will vary between manufacturers of wireless systems. An example of call control event counts totaled on an hourly basis is shown in FIG. 1.
Once the processing time of each call control event is known, and the count (i.e., # of a particular call control event per unit time, such as an hour) is measured, the processor occupancy as a percentage may be calculated for that particular call control event. The call control processor occupancy determined from a sum of processor occupancies for the relevant call control events can be verified by measuring the (total) call control processor occupancy directly using various different methods. For example, the operating system in the base station may provide hourly counts which are relatable to total processor occupancy.
As stated above, the total processor occupancy may be estimated by summing the processor occupancy for each call control event (e.g., originations, terminations, handoffs, etc.) and the no-load processor occupancy. The processor occupancy for each call control event is obtained by multiplying the real-time used per occurrence of that event and the number of occurrences of that component during the hour (e.g., determined from the count for that call control event). For example, to obtain the real-time spent on AMPS call originations during an hour, one would multiply the number of AMPS originations for the cell during the hour (from the count for AMPS originations) by the AMPS call origination processing time.
The total processor occupancy estimate in % may be written as:
{circumflex over (xcfx81)}=xcfx81nl+xcex1xc3x97(xcexa3ixcex2iANiA+xcexa3jxcex2jTNjT)xc3x97100xe2x80x83xe2x80x83(1)
where:
{circumflex over (xcfx81)} is the estimated total processor occupancy expressed as the percentage of time that the processor is busy with call processing tasks and no-load activity.
xcfx81nl is the no-load processor occupancy expressed as the percentage of time that the processor is busy with non-call processing related tasks. The no-load processor occupancy, xcfx81nl, can be evaluated by measuring processor occupancy when the call volume is low (e.g., in the pre-dawn hours).
xcex1 is a scale factor used to obtain consistent units in the occupancy calculation.
xcex2iA is the processing time for the ith AMPS call processing event, and i is an index that runs over all AMPS call processing events.
NiA is the number of occurrences per unit time of the ith AMPS call processing event, and is obtained from the appropriate count.
xcex2jT is the processing time for the jth TDMA call processing event, and j is an index that runs over all TDMA call processing events.
NjT is the number of occurrences per unit time of the jth TDMA call processing event, and is obtained from the appropriate count.
The above equation is valid for dual-mode AMPS and TDMA base stations. If the base station is AMPS or TDMA only, then the second or first summation term, respectively, will be zero. No other change to Equation 1 is necessary to handle single mode base stations.
FIG. 2 shows both the estimated processor occupancy (from Eqn. 1) and the directly measured processor occupancy counts in a base station versus the hour of the day for a 24 hour duration. It may be observed that the Equation 1 estimator is very accurate.
It is desirable among providers of wireless service to determine the call capacity of a base station for planning purposes This xe2x80x9ccall capacityxe2x80x9d is defined as the maximum calling rate (in terms of originations+terminations per hour or alternatively originations per hour) which results in a processor occupancy of xcfx81max. For the purposes of explanation, xcfx81max may correspond to the processor occupancy at which base station call setup delays are excessive. Service providers typically desire to know at what call capacity a processor occupancy of xcfx81max will be reached, because such a call capacity is directly related to available revenue per base station, as well as useful for determining when to upgrade the processor and/or split the cell. Thus, call capacity prediction based on originations+terminations per hour or originations per hour would be valuable.
Likewise, it is also desirable to predict various call control event rates, such as handoffs in or out of a base station, (also in terms of originations+terminations per hour or alternatively originations per hour) for use in system engineering of wireless communication systems. In such a case, engineering tradeoffs between the signal strength thresholds at which locates occur and processor capacity could be made. Prediction of handoff rates based of originations+terminations per hour or originations per hour would also be valuable.
The use of two alternate variables (i.e., originations+terminations and originations) may be due to the pricing structures of different service providers. For example, United States service providers may charge users for calls originated and received at a mobile unit, and are hence interested in the former metric. European service providers, by contrast, may charge users only for calls originated at a mobile unit, and are hence interested in the latter metric.
However, the occupancy estimation Equation 1 is not amenable to either capacity estimation or call control event prediction, because it depends on a large number of call processing events. Such a large number of variables is ill-suited for estimating the call capacity which will result in a processor occupancy xcfx81max. Nor can individual call processing event rates (e.g., handoffs) be predicted from Equation 1.
The present invention provides a method for predicting the percentage of time a processor in a base station is being utilized (i.e., processor occupancy) based on calling rates, given measured call control event data (e.g., origination, termination, handoff, paging, registration, and locate rates) for the base station. Constant ratios between the various call control events and the calling rates (e.g., originations+terminations) are determined for a base station. Given the actual time spent by the processor on each type of call control event and the constant ratios, the processor occupancy may be accurately predicted using call originations+terminations or call origination rates as variables.
In one embodiment of the present invention, the processor occupancy is fixed at some maximum allowable value and the call capacity of the base station (e.g., the total number of call originations and terminations which will result in that maximum processor occupancy) is determined from the relationship between processor occupancy and calling rates. This allows estimation of the number of users a base station may support prior to reaching a high processor occupancy threshold. For deployed base stations, a xe2x80x9csnap shotxe2x80x9d of current field data is used to compute the above constant ratios and validate their accuracy in processor occupancy estimation. Use of such data is feasible, because call volume typically grows relatively slowly, enabling use of data from a base station which is currently at, for example, 40-50% occupancy to predict the call volume at which the maximum allowable processor occupancy will be reached (i.e., the call capacity) in many months or several years. For base stations yet to be deployed, the above relationship enables coarse estimates of the calling capacity given some assumptions about the operating environment. Hence, the invention enables estimation of the call capacity for both base stations currently in use, and base stations yet to be deployed. Such predictions are very valuable for system planning purposes.
In another embodiment of the present invention, the handoff rate, the paging rate, the registration rate, or the locate rate is determined based only on the calling rate. This allows engineering tradeoffs between, for example, the available calling rate for a base station and corresponding handoff or locate thresholds.